Sony STR-VX4 - Page 38

SONY STR-VX Series Circuit Operation
Impedance Detector
The circuit feeds a signal to the relay
drive circuit to open the relay switch
in order to protect the speakers if ex-
cess current flows in the output stage.
When the base-emitter voltage of Q718
is raised to 0.6V, Q727 turns on.
Then, the collector voltage of Q727
goes low. Because the collector of
Q727
is
connected
to
the
base
of
Q722,
Q722 turns off, disengaging the relay
switch as shown in Figure 11.
Current Limiter
When a very low impedance speaker is
connected or extremely high output cur-
Fig. 12.
12
rent flows in the output stage, the cur-
rent limiter operates to protect the am-
plifier. If the base-emitter voltage of
Q716 is raised to 0.6V because of an
excessive voltage drop across R730,
Q716 turns on, and the voltage between
points A and B will become about 0.6V
as shown in Figure 12. This cuts off
the driver transistors (Q710, Q712) and
output transistor Q714. Likewise, if
the base-emitter voltage of Q717 is
raised to 0.6V via R728 from R731,
Q717 turns on. Then Q711, Q713 and
Q715 switch off. D705 and D706 are
connected to prevent current from
flowing from the output to the input
of the driver stage via Q716 and R727,
or Q717 and R728.
O B+ (40V)
O B— (-40V)
Current Limiter.